1. Field of the Invention
The present invention relates to strip line or microstrip directional couplers. In particular, the invention relates to sawtooth edge coupled directional couplers.
2. Description of Related Art
In FIG. 1, a conventional directional coupler 10 is depicted as a four port network having ports (1), (2), (3) and (4). Coupler 10 includes primary line 12 and secondary line 20. Primary line 12 includes coupled primary arm 14, first port feed 16 and second port feed 18. Secondary line 20 includes coupled secondary arm 22, third port bend 24, third port feed 28, fourth port bend 26 and fourth port feed 30. Power P1 is provided to the coupler at first port (1), and powers P2, P3 and P4 are measured as the output of ports (2), (3) and (4), respectively. The coupling factor CF of the coupler for coupling power that has been input in port (1) and output from port (3) is P3/P1, the directivity of the coupler is P4/P3, and the insertion loss of the coupler is P2/P1, although the coupling factor, the directivity and the insertion loss are usually expressed in decibels (dB). However, other definitions may be used. For example, the coupling factor CF may be defined to be P4/P1 (power coupled from port 1 to port 4) or P3/P2 (power coupled from port 2 to port 3) and the insertion loss of the coupler may be defined to be P1/P2.
FIG. 2 depicts a conventional microstrip transmission line, and FIG. 3 depicts a conventional strip line transmission line. Conventional coupler 10 is made from either conventional microstrip transmission lines or conventional strip line transmission lines. FIG. 4 depicts a conventional microstrip coupled transmission line pair, and FIG. 5 depicts a conventional strip line coupled transmission line pair as is formed between coupled primary arm 14 and coupled secondary aim 22 of FIG. 1.
In known couplers formed on a homogeneous medium, the solution to the well known Telegrapher""s equations (of signal propagation) applied to the transmission line pair of either FIG. 4 or FIG. S reveals that signal transmission is possible only in one of two eigenmodes that each have electrical and magnetic field components only in a direction transverse to the propagation direction (i.e., TEM modes). These two TEM modes are conveniently labeled o for odd and e for even. For coupled strip lines (FIG. 5), the characteristic impedence of the two modes are different and are given by:                     Z                              0            ⁢            e                    ,                      xe2x80x83                    ⁢          o                    =                        30          ⁢                      π            ⁢                          (                              b                -                t                            )                                                                          ϵ              r                                ⁢                      (                          W              +                                                                    bC                    f                                                        2                    ⁢                    π                                                  ⁢                                  A                                      e                    ,                    o                                                                        )                                ,          
        ⁢                  A        e            =              1        +                              ln            ⁡                          (                              1                +                                  tan                  ⁢                                      xe2x80x83                                    ⁢                  h                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            )                                            ln            ⁢                          xe2x80x83                        ⁢            2                                ,          
        ⁢                  A        o            =              1        +                              ln            ⁡                          (                              1                +                                  cot                  ⁢                                      xe2x80x83                                    ⁢                  h                  ⁢                                      xe2x80x83                                    ⁢                  θ                                            )                                            ln            ⁢                          xe2x80x83                        ⁢            2                                ,          
        ⁢          θ      =                        π          ⁢                      xe2x80x83                    ⁢          S                          2          ⁢          b                      ,          xe2x80x83        ⁢    and                      C        f            ⁡              (                  t          /          b                )              =                  2        ⁢                  ln          ⁡                      (                                                            2                  ⁢                  b                                -                t                                            b                -                t                                      )                              -                        t          b                ⁢                              ln            ⁡                          [                                                t                  ⁡                                      (                                                                  2                        ⁢                        b                                            -                      t                                        )                                                                                        (                                          b                      -                      t                                        )                                    2                                            ]                                .                    
If the power coupling factor per meter k(x) is known or calculated from first principals, the coupling factor of the coupler can be computed by integrating along the length from x=0 to x=l of the coupled transmission lines (14 and 22 in FIG. 1) as follows:
xe2x80x83CF=|∫k(x)exp(xe2x88x92jxcex2x)dx|,
where xcex2 is the propagation factor for the particular mode (odd or even) which may be different for the two modes. Similarly, directivity can be computed to be:       dir    =                  1        CF            ⁢              "LeftBracketingBar"                  ∫                                    k              ⁢                              (                x                )                                      ⁢                          exp              ⁢                              (                                                      -                    𝔱2β                                    ⁢                                      xe2x80x83                                    ⁢                  x                                )                                      ⁢                          ⅆ              x                                      "RightBracketingBar"              ,
and insertion loss IL=1xe2x88x92CF(1-dir). Typically, the directivity of the conventional coupler of FIG. 1 is about 14 dB, worst case.
FIG. 6 depicts a known sawtooth coupler. The zig-zag edge increases the power coupling factor per meter k(x) in both the primary line and the secondary line, slows the wave propagation velocity, and the directivity remains dependent on the wave propagation velocity, as well as other factors, along the primary and secondary lines.
It is an object to the present invention to provide a directional coupler that has improved directivity. It is a further object of the present invention to provide a directional coupler whose directivity is less sensitive to manufacturing process variations.
These and other objects are achieved in a directional coupler that includes a main arm and a branch arm. The main arm includes a first main sawtooth section, a second main sawtooth section and a main straight section coupled between the first and second main sawtooth sections. The branch arm includes a first branch sawtooth section and a second branch sawtooth section. The first branch sawtooth section includes a first side and a second side. The first side of the first branch sawtooth section is shaped to include a zig-zag edge and the second side of the first branch sawtooth section is shaped to include a non-straight edge. The second branch sawtooth section includes a first side and a second side, and the first side of the second branch sawtooth section is shaped to include a zig-zag edge. The zig-zag edge of the first side of the first branch sawtooth section is coupled to the first main sawtooth section, and the zig-zag edge of the first side of the second branch sawtooth section is coupled to the second main sawtooth section.
These and other objects are achieved with a method to make a coupler that includes steps of fabricating a coupler based on a first pattern, modifying the coupler, measuring a performance parameter of the coupler, and revising the first pattern to make a second pattern for use in making more couplers. The fabricated coupler includes a main arm and a branch arm. The main arm includes a first main sawtooth section, a second main sawtooth section and a main straight section coupled therebetween. The branch arm includes a first branch sawtooth section and a second branch sawtooth section. The first branch sawtooth section includes a first side and a second side wherein the first side of the first branch sawtooth section is shaped to include a zig-zag edge. The second branch sawtooth section includes a first side and a second side wherein the first side of the second branch sawtooth section is shaped to include a zig-zag edge. The zig-zag edge of the first side of the first branch sawtooth section is coupled to the first main sawtooth section, and the zig-zag edge of the first side of the second branch sawtooth section is coupled to the second main sawtooth section.